I could see if you SBR a rifle by cutting down the barrel and rethreading it. The Nitron coating could be damaged, and they would void the warranty on the barrel and possibly the gas system since it was designed for a longer barrel. Most poeple have to modify the gas plug hole size on the rifle when they shorten the barrel. So I wouldn't go that way, and instead would SBR a pistol.
If you SBR a pistol and add a stock, you are not modifying anything in the barrel or gas system. So they would have a very hard time denying you warranty repair since nothing changed except adding a stock.
Either way, once you SBR it, you will have to make sure that Sig will accept a class 3 weapon for warranty repair. With an SBR'd AR-15, the parts are readily available for you to fix it yourself or have a SOT dealer licensed to work on class 3 guns do it for you. But with class 3 weapons you are not supposed to have it out of your control unless the person possessing it is in your trust, or is a class 3 dealer. I know Sig has factory SBRs, so they should be able to accept the rifle for repairs in the event something happens.
Let us know what Sig says. If they void warranties for SBRing a pistol, then look elsewhere. Check out the Robinson Armament XCR SBR with the Mini or Micro uppers. They are three lug piston driven guns with interchangeable barrels and bolts. I own an XCR and love it. If I were going to SBR a rifle, I would not do it to my 556, but would instead do it to the XCR. You can replace the barrel in under 1 minute and put in a 10 inch barrel or any caliber you wish. They have 5.56, 6.8, and 7.62X39 barrels in all lenghts from 7.5" up to 18 inches.